The present invention relates to a system for a four-wheel drive vehicle for automatically changing the transmission from the two-wheel drive condition to the four-wheel drive condition or vice versa in accordance with the driving conditions. The present invention relates more particularly to a changing apparatus for the transmission having two-wheel drive and four-wheel drive transmitting devices, which employs an electromagnetic clutch in order to improve driving during a sharp turn.
In conventional four-wheel drive vehicles, a transmission device is adapted to drive either two wheels or four wheels by means of a dog clutch which is manually operated by a select lever.
Generally when a vehicle makes a turn, the rotating speeds of the front wheels and the rear wheels are different because of the difference between the turning radii of the front wheels and the rear wheels. However, usually a central differential gear for absorbing the difference is omitted to simplify construction. Accordingly, when a vehicle turns at a small radius, the rear wheels act to drive the vehicle at a speed higher than the speed driven by the front wheels because the rear wheels travel on a circle of a radius smaller than that of the front wheels. This will cause a so-called tight-corner or tight-turn braking phenomenon whereby the vehicle cannot be smoothly driven during a sharp turn.
It is an object of the present invention to provide an apparatus for controlling two-wheel and four-wheel drives during sharp turning which does not have the above disadvantages.